The beaches in Santa Monica are awesome. The shopping on the Promenade is nice too and only a few blocks from beach parking. Make sure you go to the Fred Segal store there. You can park on the blvd passed 2nd and there is meter parking. YOu can walk down the walkway to the beach and out on the pier.
I watched the "Best Beaches" specials on the Travel Channel and was somewhat disappointed that LAs beaches were overlooked... despite the fact that they filmed the hostess narratives at Santa Monica beach.
Santa Monica beach is my favorite 'cause of the location. Its pier is the most fun, with a small theme park on it. And there are miles and miles of sand.
Venice beach is the most "unique."
South Bay beaches (Hermosa, Manhattan, and the other one) are cool, too.